This invention relates to amplitude modulation systems and more particularly to vestigial sideband amplitude modulation systems such as are used in television broadcasting transmitters.
In efforts to use present state of the art solid state linear amplifiers in the low-level power drive stages of a television transmitter, these solid state amplifiers produce large amounts of undesired incidental RF phase modulation. By the use of a phase-to-amplitude modulation system, the video signal can be amplified, in power, with non-linear amplifiers (signal at constant level as a function of time) at the low level drive stages up to the final processing of the amplified, transmitter output amplitude modulation signal.
A phase-to-amplitude modulation system is described in a paper published in the Electronics Magazine of Sept. 1950 on pages 102 through 106. The paper is entitled "Phase to Amplitude Modulation" and is by William E. Evans, Jr. Although Evans suggests this type of system for television broadcasting, applicant knows of no use of this form of modulation in television broadcasting other than the referenced experimental use. Evans does not discuss the use of vestigial sideband filtering, and, since such filtering is conventionally done at high power levels just before coupling to the antenna, it is assumed that Evans contemplated the provision of sideband filtering at high power levels after the combiner. As will be discussed, it is desirable, particularly from the standpoint of filter cost and/or performance, that vestigial sideband filtering be done at a low power level. Further, it is desirable that all signal processing be done at a fixed frequency with translation to final television channel frequencies. This approach permits the use of a common signal processor for all television channels, lowering total cost and increasing reliability.